The present invention relates to a communication network, and more specifically to a station address testing method for preventing that stations each having the same address are duplicated in a network including a plurality of networks.
In general, in case that stations (or nodes) each having the same address are duplicated in a network, a communication frame addressed to a specific station is received by the plural stations having the same addresses. This causes a problem that the communication frame may be received by the unexpected station. This problem may also occur in a so-called "Local Area Network (LAN)" in which a communication is performed via a common transmission path. Addresses of stations connected to the transmission path must not be duplicated with each other. In other words, in case that a communication is executed in a connection-less condition, the addresses of the respective stations must not be duplicated with each other within the communication network, but must be unique to each other.
There is described a conventional method for achieving the above-described conditions for a ring LAN in the following publication; "Token Ring Access Method and Physical Later Specifications" IEEE std. 802.5-1985 (ISO/DP 8802/5).
In accordance with the above-described publication, the following two operations are described as an operation of a station (new station) newly subscribed (connected) to ring LAN (simply referred to as a "ring").
(1) A new station subscribed to, first of all, the ring (under the condition that other stations are not connected to the ring in question) becomes an active monitor (AM) irrelevant to a duplication of addresses. The active monitor generates a token for controlling a data transmission right, and also supervise a normal circulation of the token on the ring. PA0 (2) A new station subscribed to the ring after the second station (under the condition that AM is already present on this ring) performs a test (duplicated address test) whether or not a station having the same address already exists on this ring before it commences the stationary communication. This test is performed in such a way that a duplicated address test (DAT) frame where an address of a self-station has been set to a destination address is transmitted to the transmission path in accordance with the transmission control by the above-described token, and circulated around the ring. If there is another station having the same address as that of the self-station within this ring, when DAT frame is received by the above-described another station, a discriminator indicative of this fact is added and returned by the station in question. In the above-described publication, this discriminator is reflected by an address recognized display bit (AR bit). As a result, the newly subscribed station can recognize whether or not the duplicated address occurs within the ring in question by checking the discriminator in the circulated frame. PA0 (a) Under the first condition, namely under the condition where no active monitor is present in the ring in question at all, in case that a plurality of stations having the same addresses are substantially simultaneously subscribed (connected) to the same ring, these stations compete with each other in order to establish an active monitor. However, since none of these stations performs the duplicated address test, there exist the duplicated address stations in the ring in question in this case. PA0 (b) In a network where a plurality of stations are connected by a double ring constructed of a first ring and a second ring, the signal transmission direction of which is different from that of the first ring, when, for instance, a fault occurs, in case that a new closed ring is formed (a ring reduction) on both sides of a fault occurring point by a ring fold (loop back), the duplicated address stations may exist within this closed ring. This is because each of the stations connected to either the first ring, or the second ring performs the duplicated address test for the ring (e.g., first ring) to which the self-station is connected, but does not execute the duplicated address test between the self-station and another station connected to the other ring (for example, second ring). In the ring fold, the new closed ring detouring around the fault occurring point is formed by integrally forming the first and second rings. Therefore, although the duplicated address test is required between these rings, such a test is not performed in the above-described conventional method. PA0 (c) In case that a single larger closed ring is formed (ring expansion) by coupling a plurality of independent rings, for instance, in the above-described double ring, a similar problem to the above case may be provided when a plurality of rings which have been separated due to the fault are recombined by repairing the fault occurring point. That is to say, since no duplicated address test is carried out between the rings, the duplicated address stations are present in case that a plurality of rings are combined to form a single ring.
In case that the addresses are duplicated with each other, for instance, the self-station is bypassed (under the condition that the self-station is separated from the ring), so that prevention is achieved in such a manner that a plurality of stations each having the same address exist within the same ring. This duplicated address test is carried out as a part of a procedure under an initial (initializing) condition of the self-station in question.
However, in the above-described prior art, there is no sufficient care to be taken for the following three items. As a consequence, a problem exists in that a plurality of stations (duplicated address stations) having the same addresses are still present within the same ring.
The above-described problem (a) corresponds to a drawback of the prior art described in the above-mentioned publication, whereas the remaining problems (b) and (c) correspond to problem occurring in such a case that a new function is realized based upon the above prior art.